1. Field of the Invention
The present invention relates to a light emitting element array substrate used as an image sensor, a back-light of liquid crystal display and the like and also to an apparatus using such a light emitting element array substrate.
2. Description of the Related Art
As shown in FIG. 1, an LED array substrate (used as an example of light emitting element array substrate) which is constructed in accordance with the related art and which is used for light emission in image sensors and the like comprises a base 1 and a plurality of LED chips 2 arranged thereon in a line. A group of some LED chips 2 (four in this FIGURE) are connected in series with each other and with a current regulating resistor 3 to form a series circuit, as shown in FIG. 2. A plurality of such series circuits are further connected in parallel to each other. In such an arrangement, the entire LED array is energized to emit light by a source voltage V.
The characteristic of light emission in the LED array substrate 5 of such a type is shown in FIG. 3. Since the LED chips 2 are arranged equidistantly, a flat surface to be irradiated 4 will be less irradiated between each pair of adjacent LED chips 2 than directly above each of the LED chips 2. Except for the LED chips 2 at the opposite ends of the LED array substrate 5, however, a LED chip 2 can be complemented by the adjacent LED chips 2. The surface 4 can be irradiated substantially uniformly as a whole.
The opposite ends of the LED array substrate 5 become darker than the central portion thereof since the LED chips cannot be fully complemented by the adjacent LED chips 2.
If such a LED array substrate 5 is used in an image sensor, an original document will not be accurately read at its opposite ends by the image sensor. If such a LED array substrate 5 is used in a liquid crystal display, the opposite ends of the display darken and become difficult to view.
In order to overcome such a problem on the side of an apparatus incorporating the aforementioned type of LED array substrate, it has been proposed that light reflecting plates are located around an optical path from an LED array to a surface to be irradiated, as disclosed in Japanese Patent Application Nos.: Hei 3-250093 and Hei 4-20. Light from the LED array is reflected several times within the optical path to form a leveled light which irradiates the surface uniformly.
In order to overcome the same problem on the side of the LED array substrate itself, some proposals have been made, including decreasing a spacing between the extreme LED chip and the adjacent one at each end of the LED array substrate, increasing the brightness of only LED chips in an LED array substrate that are at the opposite ends and subjecting only LED chips in an LED array substrate that are at the opposite ends to more current. Thus, the surface can be uniformly irradiated by the LED array substrate as a whole.
However, these proposals have the following disadvantages: If the apparatus incorporating the LED array substrate is modified to overcome the aforementioned problems, the high manufacturing cost and a spatial limitation is brought by increasing of the number of parts. On the other hand, if the LED array substrate itself is modified to overcome the problems of the related art, decreasing a spacing between the extreme LED chip and the adjacent one at each end of the LED array substrate is troublesome in designing. The increase of brightness in the LED chips only at the opposite ends of the LED array substrate requires time to select such ones from many LED chips and this selection step makes the manufacturing cost high. When only the LED chips are subjected to more current, their service lives will be decreased.